1. Field of the Invention
The present invention relates to an organic EL (electro luminescence) element drive circuit and an organic EL display device using the same drive circuit and, in particular, the present invention relates to an organic EL display device suitable for high luminance color display, which can precisely regulate white balance on a display screen of a display device of an electronic device such as a portable telephone set or a PHS by regulating luminance of each of R (red), G (green) and B (blue) display colors regardless of smallness of dynamic range of regulation of a reference current value of each of primary colors.
2. Description of the Prior Art
An organic EL display panel of an organic EL display device mounted on a portable telephone set, a PHS, a DVD player or a PDA (personal digital assistance) and having 396 (132×3) terminal pins for column lines and 162 terminal pins for row lines has been proposed and there is a current tendency that the number of column lines and the number of row lines are further increased.
An output stage of a current drive circuit of such organic EL display panel includes an output circuit constructed with, for example, current-mirror circuits, which are provided correspondingly to the respective terminal pins, regardless of the type of drive current, the passive matrix type or the active matrix type. Incidentally, in a case of the passive matrix type drive current, the drive current has a peak value in order to emit light earlier by initially charging an organic EL element having capacitive load characteristics at light emission start time.
One of problems of the organic EL display device is that, when the voltage drive is used as in a liquid crystal display device, it is difficult to control a display because of large variation of luminance and difference in light emission sensitivity between R, G and B colors. For this reason, the organic EL display device should be current-driven. However, even when the current-drive is employed, light emission efficiency ratio of drive currents for the primary colors is, for example, R:G:B=6:11:10, which depends upon materials of the organic EL elements.
In view of this, it is necessary in the current-drive circuit for a color display that white balance is obtained on a display screen by regulating luminance of each of R, G and B colors correspondingly to materials of the EL elements for respective R, G and B display colors. In order to realize such white balance regulation, a regulation circuit for regulating luminance of each of R, G and B display colors on the display screen is provided.
Incidentally, JPH9-232074A discloses a drive circuit for organic EL element, in which each of the organic EL elements arranged in a matrix is current-driven and a terminal voltage of the organic EL element is reset by grounding an anode and a cathode of the organic EL element. Further, JP2001-143867A discloses a technique with which power consumption of an organic EL display device is reduced by current-driving organic EL elements with using DC-DC converters.
It is usual that the current-drive circuit of the organic EL display device generates drive currents for organic EL elements at respective column pins (column side terminal pins of an organic EL panel) by amplifying reference currents for R, G and B display colors and the regulation of drive-currents for obtaining white balance is performed by regulating the reference currents for R, G and B dislay colors.
In order to regulate the reference currents for R, G and B display colors, each of reference current generator circuits of a conventional drive current regulator circuit includes a D/A converter circuit of, for example, 4 bits and the reference currents for R, G and B display colors are regulated by setting a predetermined bit data for each of R, G and B display colors within a range, for example, from 30 μA to 75 μA. With the fact that various organic EL materials have been developed recently, the luminance regulation for realizing white balance, which is realizable by the D/A converter circuits, is not enough since the dynamic range of regulation is as small as 4 bits.
However, if the number of bits of the D/A converter circuit for luminance regulation of each of R, G and B display colors is increased to a value in a range, for example, from 6 bits to 8 bits in order to enlarge the dynamic range of regulation, the circuit size becomes large, so that it becomes difficult to fabricate the current drive circuits in one chip. Further, the miniaturization of a display device portion becomes impossible.